The percentage of mouse thyroids retaining function after 20 days in an allogeneic host and the percent lacking generalized infiltration were markedly increased by the use of hyperbaric oxygen during a four day culture period. Perfusion of the donor animal before thyroidotomy and the addition of fetal calf serum to the culture medium did not have a significant effect on the outcome, but the percent of grafts lacking generalized infiltration was slightly increased by adding hydrocortisone to the culture medium. The present application will pursue the following objectives: (a) improve the conditions of graft culture; (b) explore the effectiveness of agents toxic for macrophages, including oxygen, corticosteroids and sodium iodide; (c) apply these culture techniques to the transplantation of ograns other than thyroid; (d) separate and characterize the serum and lymphokine factors required for the activation of cytotoxic T-lymphocytes.